Played By: Roger Delgado, Pater Pratt, Geoffrey Beevers, Anthony Ainley, Eric Robers, Derek Jacobi, John Simm

For many of us in the real world, our greatest enemy is ourselves. Unfortunately, that's hard to make dramatically interesting. TV creators often settle for the next best thing: villains who are almost mirror images of the hero.

Embarking on its 50th season this year, Doctor Who is one of the longest-running shows in history. The Master—its ongoing primary villain—has been been around in one form or another since 1971. Throughout the Master's many incarnations, he has been the perfect foil to the good doctor.

The series writers admit that it's no mistake that the Master and the Doctor both have academic names—their minds were always meant to be their greatest weapons.